intel/fs: Do not lower large local arrays to scratch on gen7

intel/fs: Do not lower large local arrays to scratch on gen7

On gen7 and earlier the scratch space size is limited to 12kB.
By enabling this optimization we may easily exceed this limit
without having any fallback.

arb_compute_shader/linker/bug-93840.shader_test crashes with
this lowering on IVB due to exceeding scratch size limit.

Closes: https://gitlab.freedesktop.org/mesa/mesa/issues/2092
Fixes: 69244fc7
Signed-off-by: Danylo Piliaiev <danylo.piliaiev@globallogic.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>